WO2002095357A2

WO2002095357A2 - Chemical activation in electrophysiological measurements

Info

Publication number: WO2002095357A2
Application number: PCT/US2002/016122
Authority: WO
Inventors: Kirk S. Schroeder
Original assignee: Essen Instruments, Inc.
Priority date: 2001-05-21
Filing date: 2002-05-21
Publication date: 2002-11-28
Also published as: US7067046B2; WO2002095357A3; AU2002310017A1; EP1405064A2; US20020014408A1

Abstract

Systems, including apparatus and methods, for performing electrophysiological measurements on membranous samples, such as living cells, isolated cell fragments (such as organelles), and/or artificial membranes (such as vesicles). These systems preferably employ activatable or caged compounds to study ligand-gated ion channels and transporters, sequentially and/or simultaneously.

Description

SYSTEM FOR RAPID CHEMICAL ACTIVATION IN HIGH-THROUGHPUT ELECTROPHYSIOLOGICAL MEASUREMENTS

Cross-References to Additional Materials This application incorporates by reference in their entirety for all purposes the following publications: Richard P. Haugland, HANDBOOK OF FLUORESCENT PROBES AND RESEARCH CHEMICALS (6^th ed. 1996); and Joseph R. Lakowicz, PRINCIPLES OF FLUORESCENCE SPECTROSCOPY (2^nd ed. 1999).

Field of the invention The invention relates to the field of electrophysiology, wherein electrical measurements are made on biological cells and cell membranes to understand the properties and interactions of specific membrane components such as ion channels and transporters. More particularly, the invention relates to systems for performing electrophysiological measurements, typically in parallel, and typically without direct human intervention, especially on ligand-gated ion channels and/or transporters.

Background of the invention The electrical behavior of cells and cell membranes is of profound importance in basic research as well as in modern drug development. A specific area of interest in this field is in the study of ion channels and transporters. Ion channels are protein- based pores found in the cell membrane that are responsible for maintaining the electrochemical gradients between the extracellular environment and the cell cytoplasm. These channels quite often are selectively permeable to a particular type of ion, e.g., calcium, chloride, potassium, or sodium. The channels generally comprise two parts: (1) the pore itself, and (2) a switch mechanism that regulates the conductance of the pore. The switch mechanism may be controlled by transmembrane voltage changes, covalent modification, mechanical stimulation, and/or chemical ligands (e.g., through the activation or deactivation of an associated membrane receptor), among others. Ion channels are passive elements in that, once opened, ions flow in the direction of existing electrochemical gradients. Ion transporters are similar to ion channels in that they are involved in the transport of ions across cell membranes; however, they differ from ion channels in that they require energy for their function and in that they tend to pump actively against established electrochemical gradients.

Ion channels are prevalent in the body and are necessary for many physiological functions, including the beating of the heart, the contraction of voluntary muscles, and the signaling of neurons. They also are found in the linings of blood vessels, allowing for physiological regulation of blood pressure, and in the pancreas, allowing for the control of insulin release. As such, the study of ion channels is a very diverse and prolific area encompassing basic academic research as well as biotech and pharmaceutical research. Experiments on ion channels may be performed on cell lines that endogenously express the ion channel of interest ("native channels") as well as on recombinant expression systems such as the Xenopus Oocyte or mammalian cell lines (e.g., CHO, HEK etc.) that have been transiently or stably transfected to express the ion channel by well-known techniques. Electrophysiology also is performed on isolated cell membranes or vesicles as well as on synthetic membranes where solubilized channels are reconstituted into a manufactured membrane. I. Instrumentation

To date, the most useful and widely utilized tool for the study of ion channels and transporters is a technique called "patch clamping." This technique was first introduced almost 25 years ago, and consists of using a small glass capillary to function as an electrode in measuring currents and voltages from individual cells. Figure 1 shows a typical patch clamp measurement geometry. A glass capillary 2 is first heated and pulled to a fine tip. The capillary is then filled with a saline buffer solution 4 and fitted with a Ag AgCl electrode 6. The function of the Ag AgCl electrode is to provide an electrical connection to a wire via the reversible exchange of chloride ions in the pipette solution.

Through the use of a microscope and micromanipulating arm (not shown), the user finds a biological cell or cell membrane 8 containing ion channels 10 of interest and gently touches the cell membrane with the pipette. The measurement circuit is completed via the external ionic solution 12 and a second Ag/AgCl bath electrode 14. A high-impedance operational amplifier 16 senses the current flowing in the circuit, which is subsequently recorded and analyzed with a data recording system 18. The key to the function of the technique is the ability to form a high electrical resistance (~1 GD) seal between the glass pipette and the cell membrane 20, so that the current recorded by the amplifier is dominated by ions 22 flowing through the cell membrane and not by ions flowing around the glass pipette directly into the bath solution.

Once a high-resistance seal is achieved between the pipette and the cell membrane, there are many measurement configurations that the system can take, including the "whole-cell," "perforated-patch," and "inside-out" patch clamp configurations. The whole-cell voltage clamp is one of the more common configurations. In this arrangement, it is necessary to permeabilize the portion of membrane at the end of the pipette 24 so as effectively to place the pipette electrode inside the cell. This, in turn, allows for an external voltage command 26 to be placed between the intracellular pipette electrode and the extracellular bath electrode, thereby providing control of the cell's transmembrane voltage potential. The term "whole cell" is derived from the fact that, with this configuration, the instrument measures the majority of the currents in the entire cell membrane.

The electrical permeabilization of the membrane at the end of the pipette can be induced in many ways. Permeabilization often is achieved by using voltage pulses of sufficient strength and duration that the membrane inside the pipette physically breaks down. This approach is well known in the field and is commonly referred to as "zapping". Permeabilization also may be achieved by using certain antiobiotics, such as Nystatin and Amphotericin B. These antibiotics work by forming chemical pores in the cell membrane that are permeable to monovalent ions, such as chloride. Since chloride is the current-carrying ion for the commonly used Ag AgCl electrode, these antiobiotics can produce a low resistance electrical access to the interior of the cell. The advantage of the chemical technique is that the membrane patch remains intact so that larger intracellular molecules remain inside the cell and are not flushed out by the pipette solution as with the zapping technique. This approach also is well known in the field and is commonly referred to as a "perforated patch." The formation of high-resistance electrical seals enables the measurement system to detect very small physiological membrane currents (e.g., 10^"12 amp). In addition, by perforating a portion of the cell membrane either electrically or chemically, it is possible to control the voltage (voltage clamp) or current (current clamp) across the remaining intact portion of the cell membrane. This greatly enhances the utility of the technique for making physiological measurements of ion channel/transporter activity, since quite often this activity is dependent on transmembrane voltage. By being able to control the trans-membrane voltage (or current), it is possible to stimulate or deactivate ion channels or transporters with great precision and as such greatly enhance the ability to study complex drug interactions.

The development of the patch clamp technique revolutionized the field of electrophysiology, allowing for the direct electrical measurement of ion channel/transporter events in living cells, cell membranes, and artificial membranes. However, existing patch clamp techniques require the use of a skilled operator using a microscope and micromanipulating arm to record data from a single cell or membrane preparation using a small glass capillary. Typically, a recording session may take tens of minutes to complete and requires a high level of dexterity by the operator. In addition, especially in the case of drug screening, it generally is preferable to obtain a new cell sample for each different chemical entity to be tested. As such, the technique is not capable of looking at thousands of different conditions (e.g., chemical stimuli) per day, a common need in the biotech or pharmaceutical industry.

U.S. Patent No. 6,063,260 to Olesen describes a system intended to improve the throughput and decrease the fluid volume required of standard patch clamp technology. The improvement relies on using a standard HPLC autosampler apparatus integrated into a standard patch clamp arrangement to more easily inject multiple fluids samples into the measurement system. The invention claims to increase throughput by making multiple sequential fluid additions to the same biological membrane faster and easier. However, the Oleden invention is deficient in several respects. First, it does not allow for a plurality of different biological samples to be measured simultaneously. Second, it does not eliminate the labor-intensive aspects of micromanipulation involved in standard patch clamp electrophysiology. Third, it does not address cases in biological drug screening where multiple chemical reagent additions to the same biological sample are to be avoided (as in the case of high- throughput drug screening). Published PCT Application No. WO 99/66329 discusses the use of a perforated screen to conduct tests on biological materials, but the proposed system has significant, severe limitations in terms of a practical implementation. For example, all embodiments discussed in the WO 99/66329 application utilize multiple apertures per fluid well, placing reliance on the growth of confluent cell matrices to effectuate sealing of the multiple perforations formed in relatively thick material. In addition, although the published application makes reference to automation, no workable, fully integrated systems are disclosed that are capable of high throughput and reliability.

The invention may address these and/or other shortcomings by providing instrumentation that may be used for automated, high-throughput studies of ion channels. II. Channel Assays

The rapid and diverse signaling kinetics of ion channels makes their study both interesting and technically challenging. Many ion channels can be activated and then deactivated in a few milliseconds. This rapid time scale implies that the instrumentation used to study channel kinetics should have a fairly high temporal bandwidth, for example, on the order of 10 kHz. Such bandwidths are attainable, since high-bandwidth operational amplifiers are readily available. However, this rapid time scale further implies that the method of stimulating ion channel events also should be fast.

The issue of achieving a rapid stimulus deserves additional explanation and depends in part on whether the channels are voltage gated or ligand gated. Voltage gated channels are activated or deactivated by changes in transmembrane voltage, as mentioned previously. For these channels, the same electronics used to record ion channel currents also can be used to control the voltage stimulus. This type of measurement is common in the industry and is referred to as a voltage clamp. In this case, the time bandwidth of the stimulus, an electrical signal, is inherently fast enough to avoid degrading the kinetics of the voltage-gated ion channel signals. In contrast, ligand-gated channels are activated or deactivated by chemical or ligand binding. These channels may be gated by specific chemical messengers, such as the release of intracellular calcium, adenosine 3',5' - monophosphate (cyclic AMP or cAMP), or acetylcholine (ACh), among others. In some cases, the chemical activation of an ion channel is extr -cellular in its initiation, and, in other cases, the chemical activation is rntr -cellular. This implies that it is important that the compound not only can be released on the time scale of tens of milliseconds, but in some cases that the compound can be introduced within the membrane of a living cell. The invention may address these and/or other shortcomings by providing channel assays that may be used for automated, high-throughput studies of ligand- gated ion channels.

Summary of the Invention The invention provides systems, including apparatus and methods, for performing electrophysiological measurements on membranous samples, including living cells, isolated cell fragments (such as organelles), and/or artificial membranes (such as vesicles). These systems preferably employ activatable or caged compounds to study ligand-gated ion channels and transporters, sequentially and/or simultaneously.

Brief Description of Drawings

Figure 1 is a prior-art patch clamp electrophysiology configuration showing measurement geometry.

Figure 2 depicts the formation of an electrical seal between a single cell and a single hole in a substrate according to the invention. Figure 3 shows a substrate hole geometry utilizing a thin plastic film.

Figure 4 illustrates command voltage protocol and measured electrical leak resistance between a transfected CHO cell and a SiO coated kapton membrane pore.

Figure 5 shows whole cell physiological currents measured on CHO cells transfected with the voltage-gated potassium channel Kv3.2: results of a voltage sweep from -100 mN to +60 mV, as well as a voltage step protocol from -70 mN to various step voltages.

Figure 6 shows a measurement substrate comprising a polystyrene multi-well compartment adhered to a thin photo-machined plastic film. Also shown is the measurement platform (plenum), which accepts the measurement substrate during recording. Figure 7 shows a high throughput screening system depicting a complete functional measurement platform including an integrated electronics head for parallel electrical measurements, an integrated fluidics head for parallel fluid additions, an integrated UN illumination module, and a computer display and controller. Figure 8 depicts a preferred embodiment of a UN illumination technique, whereby the light energy is directed via optical fibers to the biological samples.

Detailed Description of the Invention The invention provides systems, including apparatus and methods, for performing electrophysiological measurements on membranous samples, including living cells, isolated cell fragments (such as organelles), and/or artificial membranes (such as vesicles). These systems preferably employ activatable or caged compounds to study ligand-gated ion channels and transporters, sequentially and/or simultaneously.

The systems provided by the invention may enable electrophysiological measurements to be made more quickly than with standard patch clamp techniques. In particular, in contrast to standard patch clamp techniques, in which a glass pipette is used to form a high-resistance electrical seal with a biological membrane, the systems provided by the invention preferably utilize a single, small (e.g., several micron diameter) hole in an at least substantially planar substrate to provide the sealing function. Moreover, the systems may allow cells or biological membranes to be maneuvered to the hole by fluid flow. Thus, this system may not only make the measurement easier, by reducing or eliminating the need for a direct human operator, a microscope, and or a micromanipulating arm, but it also may provide a format suitable for achieving multiple electrical seals in parallel, thereby increasing the measurement throughput of the device.

Through proper selection and processing of the substrate material, hole geometry, and attention to the way in which the biological membrane interacts with the substrate, a high-resistance electrical seal on the order of several hundred MD to 1 GD may be achieved. Preferred substrates include thin plastic films in which small holes have been photomachined using a laser. These substrates were then vacuum deposited with thin layers of glass to aid in the formation of the high-resistance seal. Silicon substrates are also described, wherein standard photolithographic/wet etching techniques are used to make the holes. In both cases, individual cells then are positioned onto isolated holes using differential pressure.

The invention further contemplates a substrate geometry that is directly applicable to the development of a high-throughput instrument whereby thousands of single cell electrophysiological recordings could be acquired in a single day. In addition, the invention encompasses an integrated electrophysiological measurement system which includes a computer controlled data collection system, an integrated electronics head for making parallel electrical measurements, an integrated fluidics head used in part to transfer test compounds into the measurement process, and/or an integrated activation system (such as a computer-controlled pulsed UV illumination module). This system, with the light source and associated light coupling to a plurality of test samples, may be used in conjunction with the other instrumentation to make effectuate high-throughput electrical measurements with respect to fast-acting, chemically activated electrophysiological events.

The system and aspects thereof are described below in more detail, including among others (I) single measurement chambers, (JJ) preferred substrate/aperture geometries, (lTJ) system architecture, and (IN) channel/transporter assays. I. Single Measurement Chambers Figure 2 depicts a measurement geometry with for a single measurement chamber, in accordance with aspects of the invention. Starting with a thin (<25 Dm thickness) substrate 28, a single hole 30 (~2 to 4 Dm diameter) is formed in the bottom of a chamber 32. An electrical circuit is implemented through the use of a Ag/AgCl sensing electrode 34 in contact with an ionic saline solution 36. A second isolated fluid chamber 38 allows fluid access to a bottom side of hole 30 in conjunction with a bath electrode 40, thereby completing the measurement circuit. The current flowing in the circuit is sensed by a high-impedance operational amplifier 42 and recorded by a computer controlled data acquisition system 44.

An important aspect of the invention is the ability to form an electrical seal 46 between the surface of substrate 28 and a biological membrane 48 without micromanipulation by a skilled technician. To achieve this, the sample such as a cell containing the membrane is placed in suspension in top chamber 36, and drawn to hole 30 through the use of differential pressure applied between bottom chamber 38 and top chamber 36. It has been found and demonstrated that once a cell reaches a properly chosen and engineered substrate, an electrical seal of several hundred MD to greater than 1 GD is achievable. Given this high seal resistance level, it is then possible to isolate and measure typical physiological whole cell currents (>50 pA) that occur when the ion channels in the cell membrane are activated. The high electrical resistance seal also allows for the ability to control the voltage of the cell, a very useful feature in analyzing ion channel activity.

To achieve voltage clamp of the membrane, an electrode must be placed in electrical contact with the inside of the cell. This requires electrically permeabihzing the part of the cell membrane 52 separating the two fluid chambers. This permeabilization has been effected in the present device in two ways: (1) voltage pulses ("zapping") generated by electrodes 34 and 40; and (2) flowing proper concentrations of antibiotics (Nystatin or Amphotericin B) in bottom chamber 38. There also are many other types of chemicals (e.g., gramicidin, ATP, valinomycin, etc.) that could be used to, provide electrical access to the cell interior. II. Preferred Substrate/Aperture Geometries

The apparatus may be used with any suitable cell, organelle, vesicle, or other membrane system. Exemplary mammalian cell lines of interest in ion channel expression systems include Chinese Hamster Ovary (CHO) cells and Human Embryo Kidney (HEK) cells. These cells have mean diameters in the range of 10-20 Dm. Optimum hole size in the substrate is governed by several considerations. Holes that are too large can allow cells to pass through the hole (as opposed to sealing) when differential pressure is applied. In addition, holes that are too large can impede formation of higher seal resistances. On the other hand, holes that are too small can produce a higher electrical access resistance to the interior of the cell once an electrical seal is formed. This higher access resistance degrades the time resolution and voltage control performance of the system. Given these trade-offs, a preferred implementation features hole diameters in the range of 2-4 Dm, although a wider range of hole diameters (e.g., 1-10 Dm) is feasible depending on cell type.

Given that the preferred hole diameter is on the order of a few micrometers, it is preferable that the unperforated substrate be thin (e.g., < 25 Dm), at least near the hole periphery. The reasons for this are several. Thick substrates introduce the problem of a very narrow pore relative to the substrate thickness, which in turn makes it more difficult to achieve fluid access to the membrane. Fluid contact is necessary to provide an electrical pathway to measure ion channel currents, as well as to provide the cell with a normal physiological environment. Also, when attempting to gain electrical access to the interior of the cell, a long narrow channel derived from using a thicker substrate will produce a higher electrical access resistance than that provided by a thinner substrate. As mentioned previously, a higher access resistance degrades system time resolution and the ability to voltage clamp the cell. In addition, any technique to machine the hole in the substrate is more difficult, time consuming, and costly when starting with a thicker substrate. As such, substrate materials utilized in these embodiments preferably had a thickness of less than 25 Dm in their entirety or at least near the periphery of the hole.

Accordingly, an important consideration of this invention is in the choice of the substrate used, the manner in which the substrate is processed to form the hole and the specific geometry utilized to make the concept workable in a high throughput instrument. With regards to the choice and manufacture of the substrate, two specific embodiments of the device have been demonstrated in our laboratory. A. Substrate Embodiment 1 -Thin Plastic Films In one embodiment, thin plastic films were used as a substrate. Two types of thin films were tested, PET (Dupont Mylar) and polyimide (Dupont Kapton), although in principle one could utilize any thin plastic film (e.g., polycarbonate, polypropylene, polyethylene, etc.). The small diameter 2-4 Dm holes were then photomachined into the plastic film using two processes. Holes were first photo-machined using a pulsed YAG laser operating at 355 nm. In this arrangement, a single laser beam drills an isolated hole, one at a time. This beam is then scanned, typically using a galvanometric mirror scanning system to raster scan the incident beam over the substrate creating an array of photo-machined holes. Such systems often employ an F-Theta lens system, which focuses as well as redirects the scanned laser beam so that the beam remains perpendicular to the target. The throughput of the scanning arrangement is thus governed by the time to drill one hole and the speed of the optical scanner. Another photomachining process implemented involved using an excimer laser operating at 248 nm. These systems work by imaging a photo-mask onto the substrate and ablating the surface where the unmasked optical energy is allowed to pass through to the substrate. Using a proper mask design, the excimer imaging process can machine multiple holes in the substrate simultaneously. This parallel machining process may provide a cost advantage in the large-scale production of such films. Data presented in this disclosure were gathered on substrates processed using the excimer laser system.

After the photo-machining process, the substrates were cleaned and subjected to a physical vapor deposition (PND) of a silicon oxide SiO₂ coating using an RF sputtering process. The process involved pumping the system down to ~ 4 x 10^"6 torr using a cryo-pump, and subsequently backfilling the chamber with 7 mtorr of Argon. The high RF field generated between two electrode plates then interacts with the Argon to produce an ion bombardment of a SiO target. The dislodged SiO then is deposited onto the thin plastic film that is placed on a rotating platter running at 20 rpm. All operations are run at room temperature. Coating thicknesses implemented were in the range of 500 to 1000 angstroms.

It was determined experimentally that the SiO₂ coating of the plastic film significantly enhanced the electrical sealing properties between the substrate and the cell membrane, increasing the seal resistance from tens of MD for the bare plastic film to resistances on the order of 1GD with the deposited glass coating. Other implementations of the coating process may be possible, such as using different thicknesses, different constituents (e.g., boron doped), and different deposition techniques (e.g., chemical vapor deposition). The specific implementation described here should not limit the scope of the invention.

Figure 3 depicts two separate examples of a cell 58 positioned over a hole 56 in a thin layer substrate 54. As shown, due to the nature of the photomachining process, the holes are larger on one side than the other; the diameter on the smaller side of the pore is in the range of 2-4 Dm. In each case a SiO₂ coating 60 is applied to the cell-side surface to improve seal formation. Both geometries have proven to be viable in achieving good electrical resistance between the cell membrane and the substrate. Figures 4 and 5 demonstrate typical whole-cell electrophysiological data acquired on CHO cells transfected with the voltage gated potassium channels Kv3.2. In this case the substrate material was Kapton, the hole was photomachined with an excimer laser (~3 Dm diameter), and the resultant substrate was coated with a 500 angstrom SiO₂ coating. The cell was positioned onto the hole in the substrate using differential pressure of approximately 5 inches of H20. After contacting the membrane, a seal resistance of approximately 1.3 GDwas measured.

Figure 4 contains two data graphs relating, to measured electrical leak resistance between a transfected CHO cell and a SiO₂ coated kapton membrane pore. The top graph represents the applied command voltage placed on the measurement electrode. As shown, the voltage sweeps from -100 mN to +60 mV (range of 160 mV) over approximately a 90-msec time course. The bottom graph represents measured current after the electrical seal was formed. As shown, the current over the same time course increased approximately 120 pA. Since the resistance of the cell membrane itself without ion channel activation is on the order of 10 GD the measured current in this example is primarily due to leak resistance. The leak resistance, which is a measure of the electrical seal between cell membrane and the substrate, is computed from the data as (160 mV / 120 pA) = 1.3 GD

To demonstrate voltage control of the cell and physiological currents, the whole-cell configuration was implemented using the antibiotic amphotericin B to permeabilize chemically the part of the membrane covering the hole. This was accomplished by flowing amphotericin B at a concentration of 200 Dg/ml to the underneath side of the hole. The mode of action of this compound is then to partition into cell membranes, where it interacts with cholesterol to form tiny channels permeable to monovalent ions. This provides a low-resistance electrical access to the interior of the cell and in turn allows for control of the transmembrane voltage over the remaining unpermeabilized cell membrane.

Figure 5 contains two data graphs relating to the physiological measurement of the Kv3.2 channel activity after the application of amphotericin B and under "whole cell" conditions. The top graph respresents the applied voltage sweep, which ranged from -100 mV to +60 mV (same sweep as that of Figure 4), providing a measure of the voltage activity of the channel. As shown, there is practically no current present until approximately 50 msec into the sweep (transmembrane voltage of -10 mV), at which time the potassium channels open and a positive current (out of the cell) is recorded. The bottom graph represents measured current generated by channel activity, where the voltage clamp was stepped sequentially for 90 msec intervals from a resting potential of -70 mV to the different respective voltages labeled on the graph. As shown, for this particular channel, current is slightly activated at a membrane potential of -20 mV, and is greatly activated at more positive potentials.

Although the data represented in Figure 4 and 5 was gathered from a single cell on a single hole, the substrate, processing, and experimental method utilized is entirely amenable to one where multiple cells could be measured in a parallel architecture.

B. Substrate Embodiment 2 - Silicon Wafers

In another embodiment, standard solid-state process techniques to produce a perforated membrane substrate. The processing started with <100> p-type silicon wafers that had been polished on both sides. After cleaning, a 4000 A layer of silicone oxide (SiO₂) was thermally grown on both sides of the wafer. This layer then was followed by a 2000 A layer of silicon nitride (Si₃O₄) and a second 4000 A layer of SiO₂, each of which was deposited using LPCVD on both sides. The front side of the wafer then was patterned with photoresist to allow for the removal of a 1 mm square section of all three oxide layers through Reactive Ion Etching (RIE). The back side of the wafer then was patterned to allow for the removal of a coincident 4 Dm diameter section of the oxides, again through a reactive ion etch.

After stripping and cleaning, an anisotropic wet etch was performed in EDP to produce a pyramidal shaped hole from the front side of the wafer (1mm square) to the oxide layers on the back side of the wafer. This resulted in a 1-Dm thick, 300- Dm square membrane of oxides with a 4- Dm diameter hole in the center. This process may be extended to produce wafer substrates exhibiting 1 or 2-dimensional patterns of hundreds to thousands of holes. Individual cells then were positioned onto the individual etched holes using differential pressure, as described previously. III. System Architecture

This section describes an exemplary system architecture enabling electrophysiological measurements to be conducted serially and/or simultaneously on a plurality of samples; see Figures 6-8. This architecture may involve the interplay of several subsystems that in concert provide the requisite functionality, including (A) a multiaperture substrate, (B) a sample-handling / fluidics system, (C) an electronics / measurement system, (D) an optical system, and/or (E) a control system, among others.

A. Multiaperture Substrate The multiaperture substrate generally comprises any mechanism having a plurality of holes or apertures about which a corresponding plurality of samples may be positioned and/or sealed for analysis. The substrate preferably has one aperture per sample, although there may be two or more apertures per sample in some configurations. The substrate also preferably allows each sample to be independently exposed to reagents, candidates, and/or other materials, for example, via separate sample wells in fluid isolation from other sample wells, at least on one side of the substrate.

Figure 6 shows an exemplary multiaperture substrate. Generally, to perform a plurality of simultaneous cell measurements, it is advantageous to replicate hole fabrication onto one substrate, preferably using the above-described technologies. There are many possible implementations of such a replication, in one or two dimensions. . In this implementation, a thin substrate 62 (e.g., Kapton film or silicon) is photomachined to form a desired rectangular array (e.g., 48 x 8, as shown, or m x n, more generally) of individual holes 64 spaced a few millimeters apart. This substrate is then joined to a multi-well fixture 66 (e.g., injection molded polystyrene), which comprises a corresponding rectangular format (48 x 8) of individual wells 68. The purpose of the multi-well fixture is to provide isolated fluidics chambers for each individual hole. The thin substrate is joined to the multi-well fixture using any suitable mechanism (e.g., by a non-toxic adhesive or ultrasonic bond), forming an electrically isolated fluid chamber on top of each isolated hole in the substrate. The entire fixture/substrate assembly will be referred to as the measurement substrate 70. B. Sample-Handling / Fluidics System

The sample-handling / fluidics system generally comprises any mechanism for adding, removing, replacing, and/or transferring samples, reagents, candidates, and/or fluids from one or both sides of one or more sample wells, sequentially and/or simultaneously.

Figures 6 and 7 shows aspects of an exemplary sample-handling / fluidics system, in accordance with aspects of the invention. Generally, o use the measurement substrate in an instrument designed for parallel simultaneous measurements, it is advantageous to be able to add suspended cells, cell membranes, microsphere beads with adherent cells, and/or other cell-derived or membranous material into each respective well of the measurement substrate. The goal is to position one isolated cell or cell construct on top of each isolated hole in each individual well. Once cells are added to each well, "cell positioning" is accomplished by applying differential pressure across the substrate to increase fluid flow through each hole. The cells or cell/bead constructs then are carried by the fluid flow to the single hole in each chamber, at which time an electrical seal can form.

The depicted embodiment uses differential pressure to lock measurement substrate 70 into position onto a plenum 72. The purpose of the plenum is to provide an airtight seal between all of measurement substrate 70 and a plenum reservoir 73 as well as to provide fluid access to the bottom side of each hole in substrate 62. The plenum is designed as a common fluidics reservoir tied to a pump system, whereas the reservoir fluid can be cycled enabling fluid constituents in the reservoir to be altered, e.g., the aforementioned addition of a chemical for electrical permeabilization of the membrane. Because of the airtight seal, the fluid in the plenum reservoir may be maintained at slightly less than atmospheric pressure, thereby introducing a differential pressure across the membrane and in turn forcing fluid flow from the top chamber through each individual hole and into the common lower reservoir. This flow causes individual suspended cells (or cell membranes) in multi-well compartments 68 to be pulled down onto individual membrane holes 64 in parallel and without direct human intervention. In addition, once the cells contact the membrane surface, the continued use of differential pressure enhances the formation of high-resistance electrical seals between the substrate material and the cell membrane.

While introducing physiological buffer solution to the underneath side of the membrane is accomplished by the plenum system, a separate fluidics system is required for the top. Fluid access to the top of the measurement substrate is convenient for the following functions:

• the introduction of physiological saline buffer to multi-well chambers 68;

the introduction of suspended cells, cell membranes, or cells adhered to beads into multi-well chambers 68; and

• the introduction of experimental chemical entities to multi-well chambers 68 for the purpose of analyzing their effect on the electrophysiology of the biological membrane.

All of the above-identified functions lend themselves to a fluidics system that aspirates fluid from a source reservoir or multiwell plate and then dispenses the same fluid in a destination reservoir. In its simplest configuration, the fluidics system may be implemented using a single pipette channel, whereby fluid is transferred from a source reservoir to one destination well at a time.

Figure 7 depicts a system architecture for the instrument, wherein the integrated fluidics head comprises a mechanical position element 74 and a twelve- channel pipette head 78. The mechanical positioning element produces the two or three-dimensional positioning of the fluidics head over the various fluid reservoirs of the system, as needed or desired. More generally, a mechanical positioning element may be used to move the fluidics head and/or one or more of the fluid reservoirs to bring these components into alignment as necessary or desired. As an example, cells, other samples, and/or reagents may be stored in one or more reagent stations 79, and potential drug candidates may be stored in solution in one or more multi-well drug plates 80. Then, at an appropriate time in the experimental cycle, these chemical entities may be transferred into the measurement substrate. In this depiction, the cells and/or candidates may be transferred for 12 wells simultaneously by aspirating fluid from the reagents and/or drug plates, respectively, and in turn dispensing the materials into appropriate measurement substrate wells 88. The fluidics architecture may be varied as necessary or desired to accommodate a given plate geometry , e.g., using a two-dimensional (n x m) multi-channel pipette, as opposed to the one-dimensional 12- position head shown. C. Electronics /Measurement System

The electronics / measurement system generally comprises any mechanism for applying and/or measuring electrical potentials and/or currents from one or more samples, in one or more sample wells, sequentially and/or simultaneously.

Figure 7 shows an exemplary electronics / measurement system, in accordance with aspects of the invention. Consistent with the aforementioned description, to make the electrophysiogical measurements on the cells, an electrical circuit must be implemented across each individual substrate hole. This may be accomplished using a sense electrode on one side of the membrane and a ground electrode on the other. For example, as depicted in Figure 7, this may involve using an electronics head element 82 consisting of 12 individual measurement probes 84 each capable of functioning as the sensing electrode for 12 individual wells of the measurement substrate simultaneously.

The head is capable of two- or three- dimensional motion, enabling it to move between the various wells of the measurement substrate as well as to a wash station 86 where the individual sensing electrodes can be washed between experimental runs. Each sensing electrode is tied to its own high impedance amplifier arrangement, consistent with that necessary for such measurements, and is located in the electronics head housing. The analog output signals for each of the respective output amplifiers is then digitized by appropriate analog-to-digital (AID) converters and transferred to computer for further processing. As with the fluidics system, there are many potential variations of the electronics architecture, e.g., using an implementation involving a larger number or array (n x m) of sensing electrodes.

Each individual circuit is completed by the addition of saline solution in each individual well of the measurement substrate above the membrane as well as by the introduction of saline solution below the membrane via a plenum 88, as described above. A common ground electrode is located in the plenum fluid reservoir, thereby completing the measurement circuit. D Activation System

The activation system generally comprises any mechanism for rapidly activating (and/or deactivating) effector compounds in one or more sample wells, sequentially and/or simultaneously. For example, the system may use suitable light from a suitable light source to activate a photoactivatable compound, and/or a suitable voltage or change in voltage from a suitable voltage source to activate a voltage- activated compound, and so on. The system preferably uses light to "uncage" a photoactivatable "caged compound" comprising a ligand, a candidate ligand modulator, and/or the like. Figures 7 and 8 show an exemplary photoactivation system, in accordance with aspects of the invention. The system includes a light source module 90 for generating light and a light coupler 92 for directing that light to one or more sample wells in measurement substrate 88. The photoactivation system and the electronics / measurement system are adapted to work in concert to photoactivate compounds and concurrently to record electronic signals such as voltages and/or currents from the same wells. This adaptation allows for the rapid and direct activation of effector compounds (e.g., through UV flash photolysis of a caged compound) and simultaneous electrical recording of time-critical, ligand-activated ion channel or ion transporter events. The light source module generally includes a light source capable of generating light that in turn is capable of or adaptable to activate the photoactivatable compound. Suitable lights sources may include continuous and/or time-varying sources, such as arc lamps, flash lamps, lasers, photodiodes, light-emitting diodes (T EDs), and/or electroluminescent lamps, among others. Preferred light sources for activating caged compounds include ultraviolet (UV) light sources, such as UV lasers^* and UV lamps. The light source module (and/or other components of the system) may control or modify one or more properties of the light outputted by the light source, such as its wavelength, intensity, polarization, and/or the like (e.g., using spectral filters, intensity filters, polarizers, and/or the like, respectively). The light source module (and/or other components of the system) also may control the timing of the delivery of the light onto the sample, including the start time and the duration of the illumination. This control may be achieved by pulsing the light source and/or by adding intervening gating optics, such as filters, shutters, acousto-optic modulators, and so on.

The light coupler generally comprises any mechanism for directing light from the light source onto one or more of the samples. Suitable light couplers may include optical fibers, free space optics, and/or evanescent wave coupling through the base of the substrate. Suitable light couplers further may include conventional optical elements such as mirrors, beam splitters, diffusers, collimators, telescopic optics, and/or the like, which may be used as appropriate in place of, or in addition to, the components previously described. The light may be directed onto the same well or sets of wells in contact with the electrical system, or a subset thereof, to facilitate coordinated activation and electrical measurement.

Figure 8 shows an exemplary photoactivation system, including a light source module 90 and a light coupler 92. The light source module includes a UV light source 94 that is controlled by a central processing unit (CPU) 96. The CPU preferably is capable of controlling the optical pulse width and intensity of the source, so that the timing, duration, and light energy of the ultraviolet exposure can be controlled automatically. The light coupler directs light from the light source onto the sample(s) via optical fibers 98 associated with electronics head element 82, such that there is a one-to-one correspondence between electrodes and illuminable wells. In summary, the addition of an activation system such as an ultraviolet light source to a high-throughput electrophysiological measurement system allows for rapid chemical stimulation via caged compound release to a plurality of measurement samples. The preferred integrated electrophysiological measurement system includes a computer-controlled data collection system, an integrated electronics head for making parallel electrical measurements, and an integrated fluidics head used in part to transfer test compounds into the measurement process. This light source, and associated light coupling to a plurality of test samples, may be used in conjunction with the system to effectuate high-throughput electrical measurements with respect to fast-acting, chemically activated electrophysiological events. The UV-source modification allows for rapid stimulation and measurement of multiple fast ligand- gated ion channel events in parallel. E. System Control / Integration

The system may include a system controller, which generally comprises any mechanism for controlling the multiaperture system, including the fluidics, electronics/measurement, and activation subsystems, or portions thereof. Figures 7 and 8 show how the multiaperture system may be controlled via an external microcomputer 96, CRT display 100, and software user interface. The system further may incorporate an embedded microcontroller , interfaced to the external PC, for controlling real-time functional aspects of the instrument, including motion control, fluidics control, and electrical data recording. The controller further may be interfaced with a three-dimensional mechanical gantry system 102 capable of independently moving the fluidics head (e.g., pipette head 78) and the electronics head (e.g., electronics head element 82). The fluidics and electronics heads may, without loss of function, independently comprise single probes, n x 1 (1 -dimensional) probes, as shown here, or n x m (2-dimensional) probes. Thus, the combination of the controller and gantry systems allows for the spatially selectable transfer of potential drug candidates to the various n x m "wells" of the multi-well measurement substrate using the fluidics head, the spatially selectable activation of caged compounds, and/or the spatially selectable electrical recording from samples using the electronics head. The integrated system further may include a system platform 104 for supporting some or all of the system components and maintaining the spatial arrangement between these components. The system components, most generally, be configured for independent and/or coordinated movement, with the individual components (or portions thereof) moveable and/or fixed, as desired, consistent with an ability to bring components into registration or alignment as needed for particular functions. For example, the fluidics head and a sample holder may be brought into register by moving the fluidics head, the sample holder, or both, using any suitable registration device or mechanism. IV. Channel/Transporter Assays The invention provides systems, including apparatus and methods, for monitoring the influence of effector agents and their modulators on membrane electrical activity. The effector agents may include activating/stimulatory agents and/or deactivating/inhibitory agents, among others, and modulators thereof. The system may be used to study any suitable electrophysiological process or event, particularly those involving ligand-gated ion channels and/or transporters. The system may be used with single samples, for example, using pipette-based or planar- substrate-based measurement devices. However, preferably, the invention may be used with multiple samples, sequentially and/or simultaneously, thereby enabling the study of fast ligand-gated electrophysiological events in a high-throughput manner.

A. Ion Channels/Transporters

The apparatus and methods provided by the invention may be used to study membrane components that are associated with or capable of bringing about measurable voltage changes and/or current flows across biological membranes. Suitable membrane components may include ion channels and ion transporters, among others, particularly ligand-gated channels and transporters. 1. Ion Channels Ion channels are membrane proteins that allow ions to flow across biological membranes, including the plasma membrane and organelle membranes. Ion channels are believed to create water-filled pores through which ions and some small hydrophilic molecules can pass by diffusion (i.e., the associated ion flow is passive, meaning that it occurs down a electrochemical gradient without requiring the input of energy.) Ligand-gated channels open or close in response to the binding, reaction, and/or other association of signaling molecules, termed "ligands." These channels may be gated by the binding of extracellular or intracellular ligands. In either case, the ligand is different than the substance that is transported when the channel opens, a. Externally Gated Ion Channels External ligands gate a variety of ion channels, including (1) ATP gated- channels, (2) glutamate-activated cationic channels, and (3) cys-loop superfamily channels. The ATP-gated channel superfamily includes the ATP2x and ATP2z receptors, among others. The glutamate-activated cationic receptor superfamily includes the NMD A, AMP A, and Kainate receptors, among others. Finally, the cys- loop receptor superfamily includes the nicotinic acetylcholine receptor, GABA_A and GABAc receptors, glycine receptors, 5-HT₃ receptors, and anionic glutamate receptors, among others. These particular channels are controlled by the ligands that appear in the names of the channels.

External ligands most often are neurotransmitters, that is, chemical substances that transmit nerve impulses across a synapse, typically to another nerve cell or a muscle cell. Exemplary neurotransmitters include acetylcholine (Ach), amino acids (e.g., glutamic acid (Glu), glycine (Gly), and gamma aminobutyric acid (GABA)), catecholamines (e.g., noradrenaline and dopamine), miscellaneous monoamines (e.g., serotonin and histamine), and peptides (e.g., vasopressin (ADH), oxytocin, Gonadotropin-releasing hormone (GnRH), angiotensin U, cholecystokinin (CCK), substance P, and enkephalins such as Met-enkephalin and Leu-enkephalin), among others. These transmitters interact in the body with channels in the postsynaptic membrane to depolarize or hyperpolarize the postsynaptic membrane, depending on the transmitter and on whether the synapse is excitatory or inhibitory, respectively. b. Internally Gated Ion Channels Internal ligands also gate a variety of ion channels. These channels may include G-protein coupled receptors (GPCRs), chloride channels, and calcium-gated potassium channels, among others. These channels generally are controlled by second messengers, which are small signaling molecules such as cyclic AMP (cAMP), cyclic GMP (cGMP), and Ca²⁺, among others. However, some of these channels are controlled by covalent modification, e.g., phosphorylation/dephosphorylation by kinases and phosphatases, respectively. 2. Ion Transporters Ion transporters are membrane proteins that use energy such as that derived from ATP to force ions or small molecules though the membrane up their electrochemical gradients. The transporters may be (1) direct active transporters, binding ATP directly and using the energy of its hydrolysis to drive active transport, or (2) indirect active transporters, using ATP indirectly by using the downhill flow of a different type of ion to drive active transport, where the gradient of the different type of ion is created by a direct active transporter, allowing another transporter to create a gradient of a different type of ion, and then using. Indirect transporters may be further subdivided into symporters and antiporters depending on whether the driving ion and the pumped ion (or other molecule) pass through the membrane in the same or opposite directions, respectively. Exemplary direct active transporters include the Na⁺/K⁺ ATPase and the H+ ATPase. Exemplary indirect active transporters include (1) symporters such as the Na+/glucose transporter, the various amino acid/Na+ transporters, and the Na+/iodide transporter, and (2) antiporters such as the Na⁺/K⁺ ATPase.

B. Activatable Compounds

Activatable compounds generally comprise any compounds, such as channel and/or transporter ligands, and modulators thereof, whose spatial and/or temporal release may be rapidly modulated by a suitable trigger, such as a change in light and/or voltage, among others.

Photoactivatible compounds, which are triggered by light, are preferred for many applications. Photoactivatable compounds are chemicals that are chemically altered such that the active nature of the compound is suppressed ("caged") until photoactivated, usually by a short pulse of ultra-violet (UV) light of wavelength in the range of 240 and 400 nm. The photolysis of such compounds is very fast and thereby can rapidly (in some cases in microseconds) release the active species of the compound. Suitable methods for producing these compounds and exemplary embodiments thereof are described in the following publication, which is incorporated herein by reference in its entirety for all purposes: Richard P. Haugland, HANDBOOK OF FLUORESCENT PROBES AND RESEARCH CHEMICALS (6^th ed. 1996).

Photoactivatable compounds may be produced by derivatizing a ligand or modulator or other compound of interest with one or more photolabile protecting or caging groups. These caging groups, which collectively form a caging moiety, are selected and/or designed to interfere maximally with the binding, activity, and/or other function(s) of the derivatized compound. These groups may be detached rapidly (e.g., in microseconds to milliseconds) by appropriate illumination (e.g., flash photolysis at < 360 nm). The groups may be incorporated into biologically active molecules using any suitable mechanism, for example, by linkage to a hetero-atom (e.g., O, S, or N) as an ether, thioether, ester (including phosphate or thiophosphate esters), amine, or similar functional group. Exemplary caging groups may include (1) α-carboxy-2-nitrobenzyl (CNB) groups, (2) l-(2-nitrophenyl)ethyl (NPE) groups, (3) 4,5-dimethoxy-2-nitrobenzyl (DMNB) groups, (4) l-(4,5-dimethoxy-2- nitrophenyl)ethyl (DMNPE) groups, and (5) 5-carboxymethoxy-2-nitrobenzyl (CMNB) groups, among others. Significantly, when intracellular application is required, the caged compound often can be made cell permeable, such that it can be loaded into the cytoplasm of the cell for rapid tra-cellular activation at a later time. Suitable photoactivatable compounds may include appropriately caged ligands, caged modulators, and the like, depending on the assay. Exemplary caged ligands include caged neurotransmitters and caged second messengers. Commericially available caged neurotransmitters include caged carbamylcholine, caged γ- aminobutyric acid (GABA), caged N-methyl-D-aspartic acid, and caged L-glutamic acid, all of which are biologically inactive before photolysis (Molecular Probes, Eugene, Oregon, USA). Commercially available caged second messengers include

9-1- caged cAMP, caged inositol 1,4,5-triphosphate, caged cADP-ribose, and caged Ca , at least several of which are membrane permeant (Molecular Probes). Exemplary caged modulators include caged ligand chelators, which can bind up ligand already present so that it no longer can bind to channels. Commercially available caged ligand chelators include caged Ca²⁺ chelators (Molecular Probes). C. Assays

The invention provides among others electrophysiological assays involving the use of activatable compounds, particularly for the study of ligand-gated membrane components such as ligand-gated channels and transporters. Activatable compounds may be especially useful in high-throughput applications, because they can be used to "introduce" compounds into solution, near an appropriate receptor, without requiring that the compound be pipetted into the solution at the time of the electrical measurement. This capability may be especially useful in systems such as the specific embodiment described above, in connection with Figures 6-8, in which rapid introduction or perfusion, on the time scale of typical channel or transporter kinetics, is difficult.

The assays may have any suitable design. Typically, caged versions of a ligand or modulator will be introduced into a system, and then activated at a suitable time using a suitable trigger, such as application of light. The electrical activity of the sample may be measured before, during, and/or after activation, so that the kinetic effects of the uncaged compound on the phenomenon of interest can be studied. Thus, in some assays, the caged compound may be a caged ligand, with the assay monitoring the effects of the ligand on a channel or transporter, typically in the presence of a candidate modulator. In other assays, the caged compound may be a caged ligand chelator or caged ligand degrader, with the assay monitoring the effects of removing the ligand from a system potentially habituated to the ligand, for example, by binding it up or destroying it. In yet other assays, the caged compound may be a caged modulator, with the assay monitoring the effects of the modulator on a system already exposed to the ligand.

The disclosure set forth above may encompass multiple distinct inventions with independent utility. While each of these inventions has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense as numerous variations are possible. The subject matter of the inventions includes all novel and nonobvious combinations and subcombinations of the various elements, features, functions and/or properties disclosed herein. Similarly, where the claims recite "a" or "a first" element or the equivalent thereof, such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. It is believed that the following claims particularly point out certain combinations and subcombinations that are directed to one of the disclosed inventions and are novel and nonobvious. Inventions embodied in other combinations and subcombinations of features, functions, elements and/or properties may be claimed through amendment of the present claims or presentation of new claims in this or a related application. Such amended or new claims, whether they are directed to a different invention or directed to the same invention, whether different, broader, narrower or equal in scope to the original claims, are also regarded as included within the subject matter of the inventions of the present disclosure. I claim:

Claims

1. An electrophysiological measurement apparatus for analyzing a biological material, comprising: a multi-well plate having a plurality of fluid chambers, wherein each fluid chamber is configured to hold a biological material to be analyzed; a thin substrate having an array of apertures in alignment with the chambers of the multi-well plate; wherein the substrate is bonded to the multi-well plate such that the chambers are open at the top and sealed at the bottom, except for the apertures; and wherein the apertures are smaller in diameter than the biological material, thereby enabling a high-resistance seal to be formed between a biological material present in each chamber and a corresponding aperture; a fluid plenum to receive the multi-well plate such that at least the substrate is immersed; a first electrode disposed in the fluid plenum; at least one second electrode adapted to fit into the top openings of the fluid chambers of the multi-well plate; a light source for illuminating one or more of the chambers so as to facilitate the rapid release of a caged compound present therein; and electrophysiological measurement circuitry in electrical communication with the electrodes.

2. The electrophysiological measurement apparatus of claim 1, wherein the light source is an ultraviolet light source.

3. The electrophysiological measurement apparatus of claim 2, wherein the light source is a laser.

4. The electrophysiological measurement apparatus of claim 2, wherein the light source is a lamp.

5. The electrophysiological measurement apparatus of claim 1, wherein a single aperture is associated with each chamber of the multi-well plate.

6. The electrophysiological measurement apparatus of claim 1, wherein the substrate is a plastic substrate having a glass coating at least in the region where the high-resistance seal is formed between the material and the substrate.

7. The electrophysiological measurement apparatus of claim 1, wherein the substrate is mylar or polyimide.

8. The electrophysiological measurement apparatus of claim 1, wherein the diameters of the apertures are in the range of 1 to 10 micrometers.

9. The electrophysiological measurement apparatus of claim 1, wherein the apertures are tapered.

10. The electrophysiological measurement apparatus of claim 1, wherein the multi-well plate is sealed to the fluid plenum, enabling a differential pressure to be applied relative to the fluid in each chamber, thereby causing the material in each chamber to migrate to a respective aperture.

11. The electrophysiological measurement apparatus of claim 1, wherein the fluid plenum includes a chemical reagent causing a biological material in each chamber to permeablize in the vicinity of the aperture.

12. The electrophysiological measurement apparatus of claim 1, wherein the electrodes are moveable.

13. The electrophysiological measurement apparatus of claim 12, further comprising a mechanism for moving the electrode into the chambers of the multi-well plate so as to automate the measurement of the material contained therein.

14. The electrophysiological measurement apparatus of claim 1, further comprising: a plurality of electrodes in alignment with a plurality of the chambers of the multi-well plate; and a mechanism for moving the electrodes into the chambers of the multi-well plate to perform simultaneous measurements on the material contained therein.

15. The electrophysiological measurement apparatus of claim 1, further comprising a system for transferring fluids from one or more sources to the chambers of the multi-well plate.

16. The electrophysiological measurement apparatus of claim 1, further comprising a mechanism for directing the light from the source to the multi-well plate in an automated and spatially selectable manner.

17. The electrophysiological measurement apparatus of claim 1, further comprising a mechanism for directing the light from the source to spatially selected wells in conjunction with a plurality of electrodes, thereby providing for the simultaneous electrical recording of the biological materials during and subsequent to the violet illumination of the materials.

18. The electrophysiological measurement apparatus of claim 1, further comprising an electronic control of optical pulse width and intensity, such that the timing, duration, and energy of the light directed from the light source onto the chambers can be automatically controlled.

19. The electrophysiological measurement apparatus of claim 1, further comprising a guide for coupling the light from the light source directly into the biological materials.

20. The electrophysiological measurement apparatus of claim 19, wherein the guide includes one or more optical fibers.

21. The electrophysiological measurement apparatus of claim 20, wherein the guide includes at least one optical component selected from the group consisting of mirrors, beam splitters, diffusers, collimators, and telescopic optics.

22. The electrophysiological measurement apparatus of claim 1, further comprising a biological material positioned in at least one of the fluid chambers.

23. The electrophysiological measurement apparatus of claim 22, wherein the biological material is selected from the group consisting of cells, organelles, and vesicles.

24. The electrophysiological measurement apparatus of claim 22, wherein the biological material includes a ligand-mediated ion channel.

25. The electrophysiological measurement apparatus of claim 22, further comprising a caged compound positioned in the at least one of the fluid chambers.

26. The electrophysiological measurement apparatus of claim 1, wherein the system is adapted to measure at least one of an electrical potential difference and a current across at least a portion of a biological material sealed across at least one of the apertures.

27. The electrophysiological measurement apparatus of claim 26, wherein the system is adapted to measure at least one of an electrical potential difference and a current across at least a portion each of two biological materials sealed across at least two of the apertures.

28. The electrophysiological measurement apparatus of claim 27, wherein the system is adapted to measure the at least one of an electrical potential difference and a current across the at least a portion each of two biological materials sequentially.

29. The electrophysiological measurement apparatus of claim 27, wherein the system is adapted to measure the at least one of an electrical potential difference and a current across the at least a portion each of two biological materials in parallel.

30. An electrophysiological measurement apparatus, comprising: a first fluid chamber containing a cell or membrane to be measured; a second fluid chamber; a thin substrate separating the two chambers, the substrate having an aperture formed therethrough which is smaller in diameter than the cell or membrane, thereby enabling a high-resistance seal to be formed between the cell or membrane and the substrate; an electrode disposed in each of the fluid chambers; and electrophysiological measurement circuitry in electrical communication with the electrodes.

31. The electrophysiological measurement apparatus of claim 30, including a single aperture formed in the substrate separating the first and second chambers.

32. The electrophysiological measurement apparatus of claim 30, wherein the substrate is a plastic substrate having a glass coating at least in the region where the high-resistance seal is formed between the cell or membrane and the substrate.

33. The electrophysiological measurement apparatus of claim 32, wherein the substrate is PET (mylar) or polyimide.

34. The electrophysiological measurement apparatus of claim 30, wherein the aperture is in the range of 1 to 10 micrometers.

35. The electrophysiological measurement apparatus of claim 30, wherein the aperture is tapered.

36. The electrophysiological measurement apparatus of claim 30, wherein a differential pressure is applied between the first and second chambers causing the cell or membrane to migrate to the aperture.

37. The electrophysiological measurement apparatus of claim 30, wherein a differential pressure is maintained between the first and second chambers until the high-resistance is formed between the cell or membrane and the substrate.

38. The electrophysiological measurement apparatus of claim 30, wherein the second fluid chamber includes a chemical reagent which electrically permeabilizes the biological membrane in the vicinity of the aperture.

39. The electrophysiological measurement apparatus of claim 30, wherein a high voltage is temporarily applied across the electrodes to permeabilize the biological membrane in the vicinity of the aperture.

40. The electrophysiological measurement apparatus of claim 30, further comprising: a plurality of first chambers forming a multi-well plate; and a substrate having a plurality of apertures; wherein the substrate is bonded to the multi-well plate such that the apertures are in alignment with the chambers thereof.

41. The electrophysiological measurement apparatus of claim 40, further comprising a light source that can be used to illuminate a selectable plurality of chambers of the multi-well plate, thereby allowing for the rapid release of a caged compound present in the selected wells.

42. The electrophysiological measurement apparatus of claim 41, wherein the light source is an ultraviolet light source.

43. The electrophysiological measurement apparatus of claim 41 further comprising a mechanism for aligning the light delivery system and the multi-well plate in an automated and spatially selectable manner, thereby automating the process of controlled caged compound release.

44. The electrophysiological measurement apparatus of claim 43, further comprising a mechanism for aligning the light with spatially selected wells in conjunction with a plurality of electrodes, thereby providing for the simultaneous electrical recording of the biological samples during and subsequent to violet illumination of the samples.

45. The electrophysiological measurement apparatus of claim 41, further comprising an electronic control of optical pulse width and intensity, such that the timing, duration, and energy of the light directed from the light source onto the chambers can be automatically controlled.

46. The electrophysiological measurement apparatus of claim 1, further comprising a light coupling mechanism for coupling the light from the light source directly into the biological samples.

47. The electrophysiological measurement apparatus of claim 46, wherein the light coupling mechanism includes a fiber optic cable or cable(s).

48. The electrophysiological measurement apparatus of claim 46, wherein the light coupling mechanism includes conventional optical means, such as through the use of mirrors, diffusers, collimators, and telescopic optics.

49. The electrophysiological measurement apparatus of claim 41, further comprising a mechanism for moving the electrode into the chambers of the multi-well plate so as to automate the measurement of the cells or membranes contained therein.

50. The electrophysiological measurement apparatus of claim 41, further comprising: a plurality of electrodes in alignment with a plurality of the chambers of the multi-well plate; and a mechanism for moving the electrodes into the chambers of the multi-well plate to perform simultaneous measurements on the cells or membranes contained therein.

51. The electrophysiological measurement apparatus of claim 41, further comprising a system for transferring fluids from one or more sources to the chambers of the multi-well plate.

52. An electrophysiological measurement apparatus for analyzing a biological material, comprising: a multi-well plate having a plurality of fluid chambers, wherein each fluid chamber is configured to hold a biological material to be analyzed; a thin substrate having an array of apertures in alignment with the chambers of the multi-well plate; wherein the substrate is bonded to the multi-well plate such that the chambers are open at the top and sealed at the bottom, except for the apertures; and wherein the apertures are smaller in diameter than the biological material, thereby enabling a high-resistance seal to be formed between a biological material present in each chamber and a corresponding aperture; a fluid plenum to receive the multi-well plate such that one side of the substrate is immersed; a first electrode disposed in the fluid plenum; at least one second electrode moveable into the top openings of the fluid chambers of the multi-well plate; and electrophysiological measurement circuitry in electrical communication with the electrodes.

53. The electrophysiological measurement apparatus of claim 52, including a single aperture associated with each chamber of the multi-well plate.

54. The electrophysiological measurement apparatus of claim 52, wherein the substrate is a plastic substrate having a glass coating at least in the region where the high-resistance seal is formed between the material and the substrate.

55. The electrophysiological measurement apparatus of claim 54, wherein the substrate is PET (mylar) or polyimide.

56. The electrophysiological measurement apparatus of claim 52, wherein the apertures are in the range of 1 to 10 micrometers.

57. The electrophysiological measurement apparatus of claim 52, wherein the apertures are tapered.

58. The electrophysiological measurement apparatus of claim 52, wherein the multi-well plate is sealed to the fluid plenum, enabling a differential pressure to be applied relative to the fluid in each chamber, thereby causing the material in each chamber to migrate to a respective aperture.

59. The electrophysiological measurement apparatus of claim 52, wherein the multi-well plate is sealed to the fluid plenum, enabling a differential pressure to be maintained relative to the fluid in each chamber until between the material in each chamber forms the high-resistance seal to the corresponding aperture.

60. The electrophysiological measurement apparatus of claim 52, wherein the fluid plenum includes a chemical reagent causing the material in each chamber to electrically permeablize in the vicinity of the aperture.

61. The electrophysiological measurement apparatus of claim 52, wherein a high voltage is temporarily applied across the electrodes to permeabilize the material in each chamber, at least in the vicinity of the apertures.

62. The electrophysiological measurement apparatus of claim 52, further comprising a mechanism for moving the electrode into the chambers of the multi-well plate so as to automate the measurement of the material contained therein.

63. The electrophysiological measurement apparatus of claim 52, further comprising: a plurality of electrodes in alignment with a plurality of the chambers of the multi-well plate; and a mechanism for moving the electrodes into the chambers of the multi-well plate to perform simultaneous measurements on the material contained therein.

64. The electrophysiological measurement apparatus of claim 52, further comprising a system for transferring fluids from one or more sources to the chambers of the multi-well plate.

65. The electrophysiological measurement apparatus of claim 52, further comprising a light source that can be used to illuminate a selectable plurality of chambers of the multi-well plate, thereby allowing for the rapid release of a caged compound present in the selected wells.

66. The electrophysiological measurement apparatus of claim 65, wherein the light source is an ultraviolet light source.

67. The electrophysiological measurement apparatus of claim 65 further comprising a mechanism for aligning the light delivery system and the multi-well plate in an automated and spatially selectable manner, thereby automating the process of controlled caged compound release.

68. The electrophysiological measurement apparatus of claim 67, further comprising a mechanism for aligning the light with spatially selected wells in conjunction with a plurality of electrodes, thereby providing for the simultaneous electrical recording of the biological samples during and subsequent to violet illumination of the samples.

69. The electrophysiological measurement apparatus of claim 65, further comprising an electronic control of optical pulse width and intensity, such that the timing, duration, and energy of the light directed from the light source onto the chambers can be automatically controlled.

70. The electrophysiological measurement apparatus of claim 65, further comprising a light coupling mechanism for coupling the light from the light source directly into the biological samples.

71. The electrophysiological measurement apparatus of claim 70, wherein the light coupling mechanism includes a fiber optic cable or cable(s).

72. The electrophysiological measurement apparatus of claim 70, wherein the light coupling mechanism includes conventional optical means, such as through the use of mirrors, diffusers, collimators, and telescopic optics.

73. The electrophysiological measurement apparatus of claim 65, further comprising a mechanism for moving the electrode into the chambers of the multi-well plate so as to automate the measurement of the cells or membranes contained therein.

74. The electrophysiological measurement apparatus of claim 65, further comprising: a plurality of electrodes in alignment with a plurality of the chambers of the multi-well plate; and a mechanism for moving the electrodes into the chambers of the multi-well plate to perform simultaneous measurements on the cells or membranes contained therein.

75. The electrophysiological measurement apparatus of claim 65, further comprising a system for transferring fluids from one or more sources to the chambers of the multi-well plate.

76. The electrophysiological measurement apparatus of claim 65, further comprising a biological material positioned in at least one of the fluid chambers.

77. The electrophysiological measurement apparatus of claim 76, wherein the biological material is selected from the group consisting of cells, organelles, and vesicles.

78. The electrophysiological measurement apparatus of claim 77, wherein the biological material includes a ligand-mediated ion channel.

79. The electrophysiological measurement apparatus of claim 77, further comprising a caged compound positioned in the at least one of the fluid chambers.

80. An electrophysiological measurement apparatus, comprising: a multi-well plate having a plurality of fluid-carrying chambers; a thin substrate having an array of apertures in alignment with the chambers of the multi-well plate; the substrate being bonded to the multi-well plate such that the chambers are open at the top and sealed at the bottom except for the apertures; and the apertures being smaller in diameter than cells or membranes typically used in performing electrophysiological measurements.

81. The electrophysiological measurement apparatus of claim 80, wherein the substrate is plastic.

82. The electrophysiological measurement apparatus of claim 80, wherein the plastic is PET (mylar) or polyimide.

83. The electrophysiological measurement apparatus of claim 80, wherein a laser is used to photomachine the apertures.

84. The electrophysiological measurement apparatus of claim 80, wherein the plastic is glass coated on the side that is bonded to the multi-well plate.

85. The electrophysiological measurement apparatus of claim 80, wherein the apertures are in the range of 1 to 10 micrometers.

86. The electrophysiological measurement apparatus of claim 80, wherein the apertures are tapered.

87. The electrophysiological measurement apparatus of claim 80, wherein the substrate is bonded to the multi-well plate using a non-toxic adhesive.

88. The electrophysiological measurement apparatus of claim 80, wherein the substrate is silicon.

89. The electrophysiological measurement apparatus of claim 88, wherein the apertures are formed using standard photolithographic techniques.

90. The electrophysiological measurement apparatus of claim 80, further comprising a light source that can be used to illuminate a selectable plurality of chambers of the multi-well plate, thereby allowing for the rapid release of a caged compound present in the selected wells.

91. The electrophysiological measurement apparatus of claim 90, wherein the light source is an ultraviolet light source.

92. The electrophysiological measurement apparatus of claim 90 further comprising a mechanism for aligning the light delivery system and the multi-well plate in an automated and spatially selectable manner, thereby automating the process of controlled caged compound release.

93. The electrophysiological measurement apparatus of claim 92, further comprising a mechanism for aligning the light with spatially selected wells in conjunction with a plurality of electrodes, thereby providing for the simultaneous electrical recording of the biological samples during and subsequent to violet illumination of the samples.

94. The electrophysiological measurement apparatus of claim 90, further comprising an electronic control of optical pulse width and intensity, such that the timing, duration, and energy of the light directed from the light source onto the chambers can be automatically controlled.

95. The electrophysiological measurement apparatus of claim 90, further comprising a light coupling mechanism for coupling the light from the light source directly into the biological samples.

96. The electrophysiological measurement apparatus of claim 95, wherein the light coupling mechanism includes a fiber optic cable or cable(s).

97. The electrophysiological measurement apparatus of claim 95, wherein the light coupling mechanism includes conventional optical means, such as through the use of mirrors, diffusers, collimators, and telescopic optics.

98. The electrophysiological measurement apparatus of claim 95, further comprising a mechanism for moving the electrode into the chambers of the multi-well plate so as to automate the measurement of the cells or membranes contained therein.

99. The electrophysiological measurement apparatus of claim 95, further comprising: a plurality of electrodes in alignment with a plurality of the chambers of the multi-well plate; and a mechanism for moving the electrodes into the chambers of the multi-well plate to perform simultaneous measurements on the cells or membranes contained therein.

100. The electrophysiological measurement apparatus of claim 90, further comprising a system for transferring fluids from one or more sources to the chambers of the multi-well plate.

101. The electrophysiological measurement apparatus of claim 90, further comprising a biological material positioned in at least one of the fluid chambers.

102. The electrophysiological measurement apparatus of claim 101, wherein the biological material is selected from the group consisting of cells, organelles, and vesicles.

103. The electrophysiological measurement apparatus of claim 102, wherein the biological material includes a ligand-mediated ion channel.

104. The electrophysiological measurement apparatus of claim 102, further comprising a caged compound positioned in the at least one of the fluid chambers.